Vertical alignment liquid crystal display device that has transistors with different switch-on resistance and method

ABSTRACT

A vertical alignment liquid crystal display device (VA-LCD) includes a display panel. The display panel includes a plurality of pixels. Each pixel unit includes a first thin film transistor (TFT), a second TFT, and a liquid crystal capacitor having a pixel electrode and a common electrode. The common electrode is applied with a common voltage, a first gray voltage is applied to the pixel electrode through a first TFT, and a second gray voltage is applied to the pixel electrode through a second TFT different from the first gray voltage, such that liquid crystal capacitor maintains two different gray voltages in a display frame time of the VA-LCD.

CROSS REFERENCES TO THE RELATED APPLICATIONS

This is a continuation application of and claims priority to U.S. patentapplication Ser. No. 14/185,342, filed Feb. 20, 2014, which is acontinuation application of and claims priority to U.S. patentapplication Ser. No. 12/454,450, filed May 18, 2009 (now U.S. Pat. No.8,692,752, issued Apr. 8, 2014), which claims priority to ChineseApplication No. 200810067271.3, filed on May 16, 2008, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a liquid crystal display device, andmore particularly to a vertical alignment liquid crystal display(VA-LCD) device and a method for driving the VA-LCD device.

2. Description of Related Art

Since liquid crystal molecules in a liquid crystal display device emitno light themselves, illumination by a light source is necessary todisplay clear and sharp text and images. By controlling the torsion ofliquid crystal molecules in the liquid crystal display device with grayvoltages, the liquid crystal display device can control the transmissionof light beams emitted from a light source, so that the liquid crystaldisplay device can display images.

Twist-nematic type liquid crystal display (TN-LCD) devices, whilecommonly used, are limited by a correspondingly narrow viewing angle,such that different colors are viewed from different angles. To overcomethe problem, a multi-domain vertical alignment liquid crystal display(MVA-LCD) device and a patterned vertically aligned liquid crystaldisplay (PVA-LCD) device have been developed. By disposing a pluralityof “<” shaped protrusions or grooves on the inner surfaces ofsubstrates, each pixel of the MVA-LCD device or PVA-LCD device isdivided into a plurality of domains. The liquid crystal molecules ofeach domain are aligned at different angles, so as to widen the viewingangle of the LCD device.

However, a long optical axis of the liquid crystal molecule has arefractive index different from that of a short optical axis of theliquid crystal molecule, generating color shift when viewed fromdifferent angles, thus the MVA-LCD device still has limited displayquality.

What is needed, is an liquid crystal display device that can overcomethe described limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

The components in the drawings are not necessarily drawn to scale, theemphasis instead being placed upon clearly illustrating the principlesof the present VA-LCD device and a method for driving the VA-LCD device.In the drawings, like reference numerals designate corresponding partsthroughout various views, and all the views are schematic.

FIG. 1 is a schematic circuit diagram of a first embodiment of a VA-LCDdevice according to the present disclosure, the VA-LCD including aplurality of pixels.

FIG. 2 is an enlarged view of a pixel of the VA-LCD device of FIG. 1.

FIG. 3 is a schematic side view of a pixel of a display panel of theVA-LCD device of FIG. 1.

FIG. 4 is a schematic plan view of the pixel of the display panel of theVA-LCD device of FIG. 3.

FIG. 5 is a waveform graph of scanning signals of the VA-LCD device ofFIG. 1.

FIG. 6 is a waveform graph of driving signals of the VA-LCD device ofFIG. 1.

FIG. 7 is a flowchart of an exemplary method for driving the VA-LCDdevice of FIG. 1.

FIG. 8 is a schematic circuit of a second embodiment of a VA-LCD deviceaccording to the present disclosure.

FIG. 9 is a waveform graph of driving signals of the VA-LCD device ofFIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made to the drawings to describe exemplaryembodiments of the present disclosure in detail.

FIG. 1 is a schematic circuit diagram of a first embodiment of a VA-LCDdevice 1 according to the present disclosure. The VA-LCD device 1includes a display panel 11, a data driving circuit 12, and a scanningcircuit 13.

The display panel 11 includes a plurality of parallel scan lines G1˜Gj,and a plurality of data lines S1˜Si parallel to each other andorthogonal to the scan lines G1˜Gj. The scan lines G1˜Gj and the datalines S1˜Si cooperatively define a plurality of pixels 113.

FIG. 2 is an enlarged view of a pixel 113 of the VA-LCD device 1 ofFIG. 1. The data line Sm−1 and the data line Sm together with the scanline Gn−1 and the scan line Gn define the m×n'th pixel 113, m and nrepresent any natural number, and 1≦m≦i, 1≦n<j, so the m×n'th pixel 113represents any pixel 113 of the VA-LCD device 1.

The m×n'th pixel 113 includes a first thin film transistor (TFT) 114, apixel electrode 152, a common electrode 142 opposite to the pixelelectrode 152 and a second TFT 115. The common electrode 142 iselectrically connected to a common voltage Vcom. A drain of the firstTFT 114 is electrically connected to the pixel electrode 152, a sourcethereof is electrically connected to the data line Sm−1, and a gatethereof is electrically connected to the scan line Gn−1. A drain of thesecond TFT 115 is electrically connected to the pixel electrode 152, asource thereof is electrically connected to the common voltage Vcom, anda gate thereof is electrically connected to the scan line Gn. The pixelelectrode 152 together with the common electrode 142 forms a liquidcrystal capacitor 116 maintaining gray voltages.

The electrical characteristic of the second TFT 115 is different fromthat of the first TFT 114. A switch-on resistance of the second TFT 115is greater than that of the first TFT 114. While the same gate voltageis applied to the gates of the first TFT 114 and the second TFT 115, andthe same voltage is applied to the drains and the sources of the firstTFT 114 and the second TFT 115, current through the first TFT 114 isgreater than that through the second TFT 115.

FIG. 3 shows a schematic side view of a pixel 113 of the display panel11 of the VA-LCD device 1 of FIG. 1. FIG. 4 shows a schematic plan viewof the pixel 113. The display panel 11 further includes a firstsubstrate 14, a second substrate 15, and a liquid crystal layer 16disposed between the first substrate 14 and the second substrate 15. Thecommon electrode 142 is disposed on a surface of the first substrate 14that faces the liquid crystal layer 16. A plurality of “<” shapedprotrusions 143 are disposed on a surface of the common electrode 142that faces the liquid crystal layer 16. The pixel electrode 152 isdisposed on a surface of the second substrate 15 that faces the liquidcrystal layer 16. A plurality of “<” shaped grooves 153 are formed inthe pixel electrode 152. Each “<” shaped groove 153 is disposed betweentwo “<” shaped protrusions 143, and each “<” shaped protrusion 143 isdisposed between two “<” shaped grooves 153.

FIG. 5 is a waveform graph of scanning signals of the VA-LCD device 1 ofFIG. 1. An interlacing scanning method is employed in the VA-LCD device1. In detail, one display frame time T of the VA-LCD device 1 is dividedinto two sequential substantially equal sub-display frame times T1, T2.In sub-display frame time T1, the scanning circuit 13 sends scanningsignals to the odd row scanning lines G1˜Gj−1 one after another; and insub-display frame time T2, the scanning circuit 13 sends scanningsignals to the even row scan lines G2˜Gj one after another. Thus, forthe m×n'th pixel 113, in the display frame time T, the time between thescanning circuit 13 sending a scanning signal to the scan line Gn−1 andsending a scanning signal to the scan line Gn is substantially half adisplay frame time T.

FIG. 6 is a waveform graph of driving signals of the VA-LCD device 1 ofFIG. 1. FIG. 7 is a flow chart of an exemplary method for driving theVA-LCD device 1. The method for driving the VA-LCD device 1 is describedas below.

In step S1, for the m×n'th pixel 113, the scanning circuit 13 sends ascanning signal to the scan line Gn−1. The scanning signal drives thefirst TFT 114 to be switched on. Then the data driving circuit 12outputs a first gray voltage Vd1. The first gray voltage Vd1 charges theliquid crystal capacitor 116 through the data line Sm and the first TFT114. As the first TFT 114 is switched off, the liquid crystal capacitor116 maintains the first gray voltage Vd1. Driven by the first grayvoltage Vd1, the electric field formed between the pixel electrode 152and the common electrode 142 inclines to four different orientationsbecause of the “<” shaped protrusions 143 and grooves 153 disposed inthe pixel 113 and one protrusion 143 being disposed between every twogrooves 153. The declining electric field make liquid crystal moleculesof the m×n'th pixel 113 align to four different orientations. Afour-domain display is achieved.

In step S2, half a display frame time T After the scanning circuit 13sending a scanning signal to the scan line Gn−1, the scanning circuit 13sends a scanning signal to the scan line Gn. When the scanning signalswitches the second TFT 115 on, the liquid crystal capacitor 116discharges through the second TFT 115. As the switch-on resistance ofthe second TFT 115 is greater than that of the first TFT 114, when thesecond TFT 115 is switched on by the scanning signal, the liquid crystalcapacitor 116 discharges incompletely. As the second TFT 115 is switchedoff, the liquid crystal capacitor 116 maintains a second gray voltageVd2 lower than the first gray voltage Vd1. Thus, another four-domaindisplay is achieved. Therefore, in the display frame time T of theVA-LCD device, the m×n'th pixel 113 achieves an eight-domain display.

In summary, for each pixel 113 of the VA-LCD device 1, in thesub-display frame time T1, the liquid crystal capacitor 116 maintainsthe first gray voltage Vd1, and in the sub-display frame time T2, theliquid crystal capacitor 116 maintains the second gray voltage Vd2.Then, the four-domain VA-LCD device 1 achieves an eight-domain display.Therefore, the VA-LCD device 1 reduces color shift, and achieves ahigher display quality.

FIG. 8 is a schematic circuit diagram of a VA-LCD device according to asecond embodiment of the present disclosure, differing from the VA-LCDdevice 1 of the first embodiment in the further inclusion of anassistant scanning circuit 24. The display panel 21 further includes aplurality of assistant scan lines Ga1˜Gaj electrically connected to theassistant scanning circuit 24. The electrical characteristic of thefirst TFT 214 and the second TFT 215 are the same. In each pixel 213, agate of the second TFT 215 is electrically connected to a correspondingassistant scan line Gan.

FIG. 9 is a waveform graph of driving signals of the VA-LCD device 2 ofFIG. 8. In operation of the VA-LCD device 2, in a display frame time Tof the VA-LCD device, the scanning circuit 23 sends a first scanningsignal Vg1 to the scan line Gn, and switches the first TFT 214 on. Theliquid crystal capacitor 216 maintains a first gray voltage Vd1. Afterhalf a display frame time T, the assistant scanning circuit 24 sends asecond scanning signal Vg2 lower than the first scanning signal Vg1 tothe corresponding assistant scan line Gan. The second scanning signalVg2 switches the second TFT 215 on, and the liquid crystal capacitor 216discharges through the second TFT 215. Since the second scanning signalVg2 is lower than the first scanning signal Vg1, the liquid crystalcapacitor 216 maintains a second gray voltage Vd2 different from thefirst gray voltage Vd1.

In summary, the four-domain VA-LCD device 2 can achieve an eight-domaindisplay, such that the VA-LCD device 2 can reduce color shift andimprove display quality. The electrical characteristics of the first TFT214 and the second TFT 215 are the same, making the VA-LCD device 2easier to fabricate.

Alternatively, the VA-LCD device 1, 2 can be any PVA-LCD device, or anyMVA-LCD device. A threefold or fourfold interlacing scanning method canalso be applied in the VA-LCD device 1. In the VA-LCD device 2, theassistant scanning circuit 24 can also send a scanning signal having ⅓display frame time delay to that sent by the scanning circuit 23. Thetime between the assistant scanning circuit 24 sending a scanning signaland that sent by the scanning circuit 23 can be between about ¼ displayframe time to about ¾ display frame time.

It is to be further understood that even though numerous characteristicsand advantages of preferred and exemplary embodiments have been set outin the foregoing description, together with details of the structuresand functions of the embodiments, the disclosure is illustrative only;and that changes may be made in detail within the principles of thepresent disclosure to the full extent indicated by the broad generalmeaning of the terms in which the appended claims are expressed.

What is claimed is:
 1. A liquid crystal display (LCD) device,comprising: a display panel comprising a plurality of pixels, aplurality of data lines, and a plurality of scan lines intersecting theplurality of data lines, the plurality of scan lines and the pluralityof data lines defining the plurality of pixels, a first pixel of theplurality of pixels comprising a first thin film transistor (TFT), asecond TFT, a liquid crystal capacitor, a pixel electrode, and a commonelectrode, the first TFT and the second TFT being directly connected tothe liquid crystal capacitor, wherein in the first pixel, the commonelectrode is electrically connected to a source of the second TFT, adrain of the second TFT is directly connected to the pixel electrode, asource of the first TFT is electrically connected to a first data lineof the plurality of data lines, a drain of the first TFT is directlyconnected to the pixel electrode, and a switch-on resistance of thesecond TFT is greater than a switch-on resistance of the first TFT, anda same gate voltage is applied to the first TFT and the second TFT,wherein in a display frame time of the LCD device, the common electrodeis applied with a common voltage, and the first pixel maintains a firstgray voltage and a second gray voltage, the first gray voltage beingdifferent from the second gray voltage, wherein a plurality of groovesare formed in the pixel electrode.
 2. The LCD device of claim 1, whereina gate of the first TFT and a gate of the second TFT are electricallyconnected to the plurality of scan lines.
 3. The LCD device of claim 1,wherein an absolute value of the second gray voltage is lower than thatof the first gray voltage.
 4. The LCD device of claim 1, furthercomprising a plurality of liquid crystal molecules aligned to differentorientations.
 5. The LCD device of claim 4, wherein the liquid crystalmolecules are aligned to four different orientations.
 6. The LCD deviceof claim 1, wherein the LCD device is a multi-domain vertical alignmentliquid crystal display device.
 7. The LCD device of claim 1, wherein theLCD device is a patterned vertically aligned liquid crystal displaydevice.
 8. A liquid crystal display (LCD) device, comprising: aplurality of data lines, and a plurality of scan lines intersecting theplurality of data lines, and a plurality of pixels defined by theplurality of scan lines and the plurality of data lines, a first pixelof the plurality of pixels comprising a first thin film transistor(TFT), a second TFT, a liquid crystal capacitor, a pixel electrode, anda common electrode, a source of the first TFT being connected to a firstdata line of the plurality of data lines, a drain of the first TFT beingconnected to the pixel electrode, a source of the second TFT beingconnected to the common electrode, a drain of the second TFT beingconnected to the pixel electrode, the first TFT and the second TFT beingdirectly connected to the liquid crystal capacitor, a switch-onresistance of the second TFT being greater than a switch-on resistanceof the first TFT, and a same gate voltage is applied to the first TFTand the second TFT, wherein in a first period of a display frame time ofthe LCD device, one of the data lines applies a first gray voltage tothe pixel electrode via the first TFT and the liquid crystal capacitormaintains the first gray voltage in the first period, wherein in asecond period of the display frame time of the LCD device, the liquidcrystal capacitor discharges through the second TFT and the liquidcrystal capacitor maintains a second gray voltage different from thefirst gray voltage in the second period, wherein a plurality of groovesare formed in the pixel electrode.
 9. The LCD device of claim 8, whereina gate of the first TFT and a gate of the second TFT are electricallyconnected to the plurality of scan lines.
 10. The LCD device of claim 8,wherein an absolute value of the second gray voltage is lower than thatof the first gray voltage.
 11. The LCD device of claim 8, whereinfurther comprising a plurality of liquid crystal molecules aligned todifferent orientations.
 12. The LCD device of claim 11, wherein theliquid crystal molecules are aligned to four different orientations. 13.The LCD device of claim 8, wherein the LCD device is a multi-domainvertical alignment liquid crystal display device.
 14. The LCD device ofclaim 8, wherein the LCD device is a patterned vertically aligned liquidcrystal display device.